Vidgelmir is among the longest and largest lava caves of the world (1585 metres), which have already been discovered. Its volume is about 148.000 m³.  It is, like the caves Surtshellir and Stefanshellir, situated in the lava field Hallmundarhraun, about 2 km to the southeast of the farm Fljotstunga in the Hvitarsida County. Its only entrance is a short distance from its northernmost end. Sometimes the narrow entrance is blocked with ice and the cave has been closed for years on end. After having squeezed through the narrow entrance the cave becomes wide and high and in many places the floor is quite even.

The traces of human habitation of the cave are now kept in the National Museum in the capital.  Archaeologists have dated them back to the Viking Age. Most of the beautiful stalactites and lava ropes have been broken and damaged, which is the reason, why the cave is locked and not open to the public without someone from the farm accompanying the visitors. Lava stalactites and ropes are created when the rest of the lava flows are leaving their courses by the still molten lava dropping from the ceiling and being stretched along the walls of the tubes (later caves). Traces of human habitation have been found in the cave. The entrance of the cave is accessible by 4wd vehicles.